1. Field of the Invention
The present invention relates generally to shock absorber assemblies for vehicles.
2. Background Information
One well known type of vehicle shock absorber consists of a cylinder filled with a shock absorbing or damping liquid, which is substantially incompressible, and a piston slidably mounted in the cylinder and connected to a piston rod which extends out of one end of the cylinder. The shock absorber is connected between the vehicle frame and wheel assembly, such that when the wheel hits a bump or other obstacle, the piston will slide into the cylinder with fluid on one side of the piston flowing through orifices in the piston to the other side to allow the piston movement. This is known as the compression stroke. The impact is damped by the resistance of the fluid to the piston movement, and the degree of damping will be determined, at least in part, by the size of the orifices. The piston is typically biassed in the opposite direction via a spring, so that when the wheel has moved over the obstacle or bump, the spring urges the piston back out of the cylinder in a so-called rebound stroke in which fluid flows through orifices in the piston in the opposite direction, with the rebound being similarly damped.
There are some problems with this basic arrangement. If the rebound is too slow, the shock absorber may not have fully recovered before a subsequent bump or obstacle is hit by the wheel, causing a hard impact felt by the occupants of the vehicle. A series of successive bumps or obstacles, as is common when driving "off-road" or "all terrain" vehicles, may give rise to a problem known as "packing" in which the piston recovers progressively less and less and eventually you have no impact damping at all, with the wheel leaving the ground after a bump and impacting hard with the next bump.
This problem can be partially overcome by making the rebound faster. Some shock absorbers provide a degree of rebound control by having a hollow piston rod with a valve rod mounted in the piston rod, the valve rod being movable by a manual adjustment mechanism to control the size of one or more rebound orifices in the piston rod. The valve rod is moved axially along the piston rod to increase or decrease the size of the rebound orifices, increasing or decreasing the rate at which fluid can flow between the opposite ends of the cylinder during rebound. Thus a faster rebound can be provided by making the rebound opening or openings larger. However, the rebound must be set manually prior to driving the vehicle, and if the rebound is too fast, the action will be uncomfortable for the vehicle occupants as the vehicle will spring up very quickly following an impact. With a constant rebound setting, the rebound will always be too fast in some situations and too slow in others.